In the course of a study of fasting plasma amino acid levels in children and adults, several unidentified ninhydrin-positive materials were observed to be present consistently on the recordings of the amino acid analyses. The same materials are also present in plasma filtrates of fasting rabbits and cows, in approximately the same amounts as in those prepared from human plasma. Isolation and identification of some of those substances is being undertaken, with the aim of determining their manner of formation and possible physiological functions. Cow blood is being used as a starting material for this work.